diff --git a/index.html b/index.html
index a2bea39..8d69113 100644
--- a/index.html
+++ b/index.html
@@ -792,15 +792,20 @@ function showTab(tabId){
 }
 
 /* ---------- helpers ---------- */
-function seg(el){ el.querySelectorAll("button").forEach(b=>b.onclick=()=>{
-  el.querySelectorAll("button").forEach(x=>x.classList.remove("on")); b.classList.add("on");});}
-seg($("segPx")); seg($("segLen"));
+function seg(el){
+  if(!el) return;
+  el.querySelectorAll("button").forEach(b=>b.onclick=()=>{
+    el.querySelectorAll("button").forEach(x=>x.classList.remove("on"));
+    b.classList.add("on");
+  });
+}
+document.querySelectorAll(".seg").forEach(el=>seg(el));
 document.querySelectorAll(".sw").forEach(s=>s.onclick=()=>{
   s.classList.toggle("on");
-  if(s.id==="swRouter") $("routerCfg").style.display = s.classList.contains("on") ? "block" : "none";
+  if(s.id==="swRouter" && $("routerCfg")) $("routerCfg").style.display = s.classList.contains("on") ? "block" : "none";
 });
-const segVal=el=>el.querySelector("button.on").dataset.v;
-const swOn=id=>$(id).classList.contains("on");
+const segVal=el=>(el && el.querySelector("button.on")) ? el.querySelector("button.on").dataset.v : "";
+const swOn=id=>$(id) ? $(id).classList.contains("on") : false;
 
 /* ---------- appearance settings (accent color, density, page size) ---------- */
 const ACCENT_PRESETS = ["#22d3ee","#e879f9","#4ade80","#fbbf24","#f87171","#818cf8","#fb923c"];